don't attempt to save large XMP to JPG

jpeg_write_marker() with some libjpeg versions will throw a fatal
error with large chunks.

To write >64kb XMP it you need to parse the whole XMP object,
pull out the most important fields, code just them into the main
XMP block, then write any remaining XMP objects into a set of
extended XMP markers.

http://wwwimages.adobe.com/content/dam/Adobe/en/devnet/xmp/pdfs/XMPSpecificationPart3.pdf

Instead, just warn and skip large XMP blocks.

see https://github.com/libvips/libvips/issues/1372

ChangeLog

@@ -1,5 +1,6 @@
 9/7/19 started 8.8.2
 - better early shutdown in readers
+- don't attempt to save large XMP to jpeg [tnextday]
 
 24/5/19 started 8.8.1
 - improve realpath() use on older libc

libvips/foreign/vips2jpeg.c

@@ -88,6 +88,8 @@
  * 	- fix a leak with an error during buffer output
  * 19/4/19
  * 	- fix another leak with error during buffer output
+ * 19/7/19
+ * 	- ignore large XMP
  */
 
 /*
@@ -285,6 +287,22 @@ write_xmp( Write *write )
 		(void *) &data, &data_length ) )
 		return( -1 );
 
+	/* To write >64kb XMP it you need to parse the whole XMP object, 
+	 * pull out the most important fields, code just them into the main 
+	 * XMP block, then write any remaining XMP objects into a set of 
+	 * extended XMP markers. 
+	 *
+	 * http://wwwimages.adobe.com/content/dam/Adobe/en/devnet/xmp/pdfs/ \
+	 * 	XMPSpecificationPart3.pdf
+	 *
+	 * jpeg_write_marker() with some libjpeg versions will throw a fatal 
+	 * error with large chunks.
+	 */
+	if( data_length > 60000 ) {
+		g_warning( "%s", _( "VipsJpeg: large XMP not saved" ) );
+		return( 0 );
+	}
+
 	/* We need to add the magic XML URL to the start, then a null
 	 * character, then the data.
 	 */